Marine structures are frequently moored to the sea bottom by anchors connected to the structure by a spread of mooring lines. Drilling rigs are illustrative of such marine structures for which the mooring system is both critical and very expensive. With the greater emphasis on offshore exploration, drilling rigs will now be required to operate within water depths of up to and beyond 3,000 feet and in very severe environments. The mooring system is designed to maintain the rig within certain horizontal limits from the center line of the well. Dynamic anchors are presently employed because they increase their holding power with horizontal pull provided by the rig, and because such anchors become deeply buried in soft bottoms.
To prevent an uplifting force from becoming exerted on and unseating the anchor, a sufficient length of mooring line must be deployed, considerably greater than the water depth. Maximum holding power is obtained when the fluke angle is set at approximately 30.degree. for sandy bottoms and 50.degree. for muddy bottoms.
Mooring lines consisting of wire rope, chain, or a suitable combination of rope and chain have been successfully used in proportions depending on several factors which include: expected mooring line loads, water depth, handling equipment, storage facilities on board the drilling rig, and types of working boats available for assisting the rig during the deployment and retrieval of the mooring system.
The location of each anchor in a mooring system is marked by a surface or marker buoy connected to the anchor by an anchor cable, known as a pendant line. A submerged assist or spring buoy can also be connected to the pendant line to provide thereto additional buoyancy and protection. In practice, a portion of the pendant line drags over the sea bottom, especially when the surface buoy becomes subjected to strong winds. Abrasion resulting from such dragging can cause a pendant line to prematurely fail.
Various shackles are used to join sections of pendant lines, to attach chain to wire rope, and to attach chain or wire rope to anchors. When long pendant lines are reeled in, the various connecting devices have a tendency to squash and crush the wire rope. After considerable use, such chain links and/or wire rope will rupture, resulting in the loss of one or more pendant lines. The work boat must "chase" the mooring line attached to the anchor whose pendant line fails in order to first locate and then retrieve the lost anchor.
Each pendant line is constructed in a manner similar to a mooring line, above described. A typical pendant cable for very deep water operation, usually will consist of wire rope with a diameter of over 13/4 inches. To hold such a cable erect may require in excess of 20,000 lbs. of net buoyancy.
For a water depth of about 1500 feet, the pendant line may have a length of more than 1800 feet while the subsurface spring buoy may have a structural weight on the order of 13,000 lbs. and a cross-sectional area of about 80 square feet. The marker surface buoy for this particular pendant line is of comparable size to the subsurface spring buoy. For 3,000 feet of water depth, the numerical values would be nearly double of the corresponding numbers given for the 1500 feet water depth. It will be appreciated therefore that the pendant cables and their associated buoys consume a great amount of steel and rig storage space, as well as constitute one additional variable load.
Also, in order to move from one water depth to another, there is a need to make adjustments in the lengths of the pendant lines, all of which is time consuming and expensive. Frequent manipulations of the pendant lines by the crews of the rig and of the work boat contribute to their rapid deterioration. Thus in the Gulf of Mexico, the average life of a pendant line is about three years, while in the North Sea it is considerably less.
It is a primary object of the present invention to eliminate the need (1) for long pendant lines and their associated buoys, thereby considerably reducing the overall cost of rig construction, the cost of anchor deployment, and at the same time increasing the safety of operations, especially in severe environments; (2) for surface and spring buoys; and (3) for making adjustments in the lengths of the pendant lines. It is another object to provide a mooring method which is particularly adapted for severe environments and which reduces the time and difficulty in running mooring lines and anchors.